1. Field of the Invention
The invention relates to a power trowel having an improved trowel blade pitch adjust assembly. The invention additionally relates to a method of operating a power trowel having a counterbalanced trowel blade pitch adjust assembly.
2. Discussion of the Related Art
Power trowels, often known as concrete finishing machines, are widely used for leveling and famishing concrete or the like. A typical power trowel machine includes a frame on which are mounted trowel arms projecting radially outwardly from a common hub which in turn is driven to rotate by an engine. Generally planar trowel blades are mounted on the trowel arms. The trowel blades rest directly on the surface to be finished and support the entire machine's weight. The machine is guided by a tubular machine handle extending upwardly and rearwardly from the frame.
The inclination or pitch of the trowel blades of many power trowels can be adjusted "on the fly" to optimize finishing. Specifically, the pitch is adjusted between each pass as the machine makes several passes over the concrete surface as the concrete hardens. In the initial pass, when the concrete is still very wet and plastic, the blade pitch is usually adjusted to be substantially parallel with the concrete surface so that the blades lie flat upon the surface and spread the machine's weight over a maximum surface area. In subsequent passes, as the concrete hardens and becomes less plastic, the blade pitch is progressively increased so that the machine's weight is spread over a smaller surface area with a resultant increase in pressure.
The typical power trowel blade pitch adjust assembly comprises a thrust collar that cooperates with a yoke pivotally secured to the machine's frame to pivot the trowel blades. A tension cable interconnects the yoke with a pitch adjust actuator--typically a screw handle that is located at the remote end of the tubular machine handle. Rotation of the screw handle adjusts the yoke's angle to move the thrust collar up or down a corresponding amount thereby to provide the desired trowel blade pitch adjustment. A power trowel having this type of blade pitch adjust assembly is disclosed, e.g., in U.S. Pat. No. 2,887,934 to Whiteman.
The typical power trowel weighs several hundred pounds. Since the entire weight of the machine rests on the trowel blades, blade pitch adjustment must be performed against a considerable resistive force imposed by the trowel blades through the cable. The threads of the traditional screw handle must have an extremely fine pitch to permit blade pitch adjustment using a reasonably low level of operator effort. Consequently, the blade pitch may be adjusted only very slowly.
It has been recognized that blade pitch adjustment can be effected more rapidly and with less effort if a counterbalancing mechanism were to be incorporated into the blade pitch adjust assembly that offsets the force imposed on the pitch adjust actuator by the weight of the machine resting on the trowel blades. For example, U.S. Pat. No. 4,673,311 to Whiteman, Jr. (the Whiteman, Jr. patent) proposes a lever-operated trowel blade pitch adjust assembly having a counterbalancing mechanism that is located within the tubular machine handle and that applies a counterbalancing force to the control lever thereby to permit blade pitch adjustment with substantially less force than otherwise would be required.
The Whiteman, Jr. patent discloses two primary embodiments, both of which employ a coil spring to facilitate operation of the control lever. In Dne embodiment, the spring surrounds the tension cable and is located within the same tubular machine handle that supports the control lever. The spring and cable of this embodiment both act on a common slide block that, in turn, is attached by a connecting rod to a portion of the control lever positioned adjacent the control lever's pivot point. In the second embodiment, the counterbalancing spring is (1) located within a separate tube carried on the underside of the tubular machine handle, and (2) bears at its upper end against a movable block that can slide within the tube. A second tension cable, disposed generally in parallel with the blade pitch adjust cable, interconnects the movable block with the control lever. In a variation of this embodiment, the spring acts on an upper slide block that is connected to a more conventional rotatable screw handle. According to the text of the Whiteman, Jr. patent, the counterbalancing force of this embodiment permits the screw handle to have a very large pitch so that the entire range of cable movement can be traversed conveniently with just a few turns of the screw handle.
In all embodiments, the force imparted by the coil spring is sized to be comparable to the force applied by the cable, thus substantially reducing the effort required to actuate the control lever. As a result, the control lever can be moved to any pivotal position without requiring the operator to apply any significant force.
The blade pitch adjust assemblies disclosed in the Whiteman, Jr. patent exhibit several drawbacks and disadvantages.
For instance, the "comparable force" imposed by the relatively large coil springs is higher than optimal in at least two respects. First, most operators expect some resistance to a mechanically-operated device such as a control lever and might suspect that the blade pitch adjust assembly is not operating properly if they do not encounter some resistance to control lever movement. Operators may also experience difficulty "fine tuning" blade pitch adjustment unless they encounter sufficient resistance to control lever movement to prevent unintended control lever overtravel. Second, the resistance to control lever movement avoids the detrimental effects that otherwise could occur if the biasing force imposed by the spring is too large. For instance, if the control lever is released under conditions in which the tension imposed by the blade pitch cable are reduced or even absent (such as when the power trowel is not resting on the ground), the counterbalancing spring tends to snap the control lever back towards the operator. This snap-back may occur with such force that the operator's fingers are pinched between the control lever and the machine's guide handle.
Moreover, disadvantages arise due to the locations of the coil springs within the tubular machine handle of the Whiteman, Jr. patent. The springs necessarily are connected to the control lever indirectly via a linkage mechanism. The coil springs therefore are inefficient and must be oversized to provide the desired counterbalancing effect. Moreover, because the coil springs are relatively long and cumbersome and hence must be placed inside the tubular machine handle, the blade pitch adjust assembly is relatively difficult to retrofit onto an existing power trowel.
In addition, all of the embodiments of the Whiteman, Jr. patent that employ a pivotal control lever as a blade pitch adjust actuator employ a secondary release system or latch that must be released before the lever can be adjusted. This secondary release mechanism includes a stationary latching gear and a lock that must be released from the gear before the lever can be adjusted. The lock takes a form of a spring-biased locking dog that is connected to a transverse finger. The finger must be manually lifted to raise the locking dog away from the locking gear before the control lever can be pivoted. Blade pitch adjustment is relatively difficult to effect because the operator must simultaneously impart a linear force to retract the locking dog and a pivoting or rotative force to pivot the lever.
In addition, the lever is difficult to access by many operators because it extends within a vertical plane bisecting the tubular machine handle. It would be more accessible if it were rotated some either to the right or to the left of this plane to facilitate access by the operator's dominant hand.